The invention relates to an apparatus for capacitive electrical detection or measurement with respect to a medium, substance, material or object in a solid or fluid form. In many practical applications the media of interest comprise water, e.g. in the form of moisture in the substance or material concerned.
The apparatus is of the type comprising at least one electrode having a given surface extent and an oscillator circuit the output frequency of which is a function of the capacitance being associated with the electrode and the dielectric constant of the medium, and taken as a basis for the detection or measurement. The measurement principle is well known per se, as will appear e.g. from the book xe2x80x9cInstrumenteringsteknikkxe2x80x9d by Odd Arild Olsen, Tapirs forlag, 1988, pages 141 and 245. Examples of specific applications are given in McGraw-Hill Encyclopedia of Science and Technology, 7th addition, volume 10, page 35 and volume 14, page 278.
There exist various earlier practical proposals and applications based on the known principle, where there is provided a capacitor in a relatively conventional sense, usually with plate-shaped electrodes between which there is received a dielectric in the form of the medium, substance or material concerned, that is subject to detection or measurement. In this connection there is reason to note that different media can have rather different dielectric constants, e.g. water having the value 80, oil the value 3 and air the value 1.
The object of this invention is to provide a novel apparatus being based upon the above principle, but having substantially expanded fields of application as well as an increased accuracy and sensitivity, as it is required when measuring e.g. pollution or moisture and likewise in demanding level measurements. As regards detection or measurement of pollution there can e.g. be the question of oil in water or water in oil. Also when level measurement is concerned the two media water and oil are of much interest in this connection.
As will appear from the following description there is here the question of detecting or measuring very small capacitance changes, and as explained in the following description it is a particular purpose to make possible exact measurement when such small capacitance changes occur. In this connection it has decisive significance that sources of error being present, are taken into account, such as temperature variations in the dielectric constant of water.
On this background and taking as a starting point an apparatus of the type mentioned at the beginning of this description, the novel and specific features according to the invention in the first place consist therein that the effective spacing of the electrode from a counter-electrode or from ground is of similar or substantially higher order of magnitude as seen in relation to the surface extent of the electrode, and that a conductor connecting the electrode to the oscillator circuit has a length that is a small fraction as seen in relation to surface extent of the electrode. More closely defined this involves in other words that the connecting conductor essentially represents a capacitance that is substantially smaller than the capacitance of the electrode.
In some embodiments according to the invention there will be incorporated a counter-electrode in the apparatus itself, whereas other embodiments do not comprise such a counter-electrode or another electrode in the apparatus, so that the environment or xe2x80x9cgroundxe2x80x9d will act as a xe2x80x9ccounter-electrodexe2x80x9d. In all cases there is here the question of a form of xe2x80x9ccapacitorxe2x80x9d that is fundamentally different from a capacitor in the usual sense. This is seen in the first place therein that here there is the question of a very xe2x80x9copenxe2x80x9d configuration where, in contrast to what applies to common capacitors, there does not exist any relatively small or directly measurable distance between two electrode plates, the surface extent or area of which is regularly of a substantial magnitude as seen in relation to the electrode or plate spacing. The apparatus according to the invention involves so to speak an inverted relationship between the effective spacing mentioned and the surface extent of the electrode. These geometrical relationships or dimensional relations will appear more closely from the exemplary embodiments described below with reference to the drawings. The same applies to the condition that the length of the conductor connection between the electrode and the oscillator circuit as mentioned, is very small compared to the surface extent of the electrode. Whereas the dimensions of the electrode in most embodiments as a rule can be reasonably measured in centimetres, the connecting conductor typically has a length that most suitably can be measured in millimetres, such as from a couple of millimetres up to a maximum or perhaps 10 millimetres.
As regards the capacitance relationship mentioned, i.e. between the connecting conductor and the actual electrode, it is to be noted additionally that the relationship applies both when the medium concerned influences the capacitance and when only air is present.
An additional factor associated with the particular electrode configuration being here of basic interest, is that the medium, substance, material or object to be subjected to detection or measurement, must not necessarily be brought close to the electrode or a possible counter-electrode in the apparatus. In certain embodiments there is obtained a desired larger penetration depth in media of interest, by locating or conveying it at some distance from the electrode, so that the capacitive or electrostatic field will be influenced by a larger volume of the medium during the detection or measurement process. Thus, the measurement will be more representative of larger proportions of the volume or bulk of the medium.
Although oscillator circuits based on various principles can be employed in this context, it is preferred according to the invention to employ an LC-oscillator where there is incorporated an air core coil and a temperature-sensing element in good thermal contact with the air core coil, for temperature compensation of the oscillator circuit. This type of oscillator-has been found to be particularly stable and to have a small sensitivity to sources of error, whereby in particular temperature compensation has much interest in the apparatus according to the invention. It will also be possible then to generate a temperature measurement signal that as such can also be utilized for other purposes than the direct temperature compensation of the oscillator circuit as mentioned above. Such utilization can take place with advantage in a microprocessor or computer, e.g. a PC, that also receives the output frequency from the oscillator circuit. Subsequent to a suitable and desired processing of the signals in the microprocessor or the computer, this delivers a resulting detection or measurement signal that can then be conveyed to an alarm, recording or display device. Such devices can also comprise additional data processing for a favourable presentation or utilization.
As already mentioned above, measurements or detection of interest can in many cases comprise water or moisture in the medium or object concerned. In such applications it is preferred according to the invention that the fundamental frequency of the oscillator circuit is within the range 10-100 MHz, preferably between 15 and 30 MHz. At such a choice of fundamental frequency the apparatus will be particularly well adapted for a high sensitivity with respect to water content, which is related to the fact that the loss factor for water has a pronounced minimum at the frequencies mentioned. As known the loss factor in this connection represents quite a different effect from the dielectric constant as discussed above. As also mentioned the dielectric constant, in particular for water, is very temperature dependent and according to the invention it is possible to compensate also for this by means of suitable processing in the microprocessor or computer mentioned. For illustration it can be mentioned here that the dielectric constant for water is about 80 at a temperature of +25xc2x0 C., whereas the value is about 40 at a temperature of +80xc2x0 C.
Among the important features contributing to a high sensitivity and good measuring results by means of the apparatus according to the invention, there is involved an advantageous calibration of the oscillator frequency based upon at one hand air and preferably dry air as a dielectric for the electrode, and at the other hand preferably another calibration with water as a dielectric for the electrode. In embodiments where several separate electrodes are incorporated, e.g. when level measurement is contemplated, such frequency calibration will be performed individually for each separate electrode. In fact such calibration will also correct for variations in components or parameters of the oscillator circuits, so that the electrode and the associated oscillator circuit in this respect will be treated as an integral unit. As it will also appear from the following description, there can moreover according to the invention, be provided for a similar calibration process when the apparatus has been put into operation, in particular in form of a repeated and adaptive calibration procedure during the whole operational time or life time of the measuring apparatus. In particular in the case of such apparatus where there is incorporated a number of individual electrodes as mentioned above, calibration as discussed here will be important, since each electrode-oscillator probe initially can have varying properties or parameters with respect both to sensitivity and temperature coefficient as well as fundamental frequency. All such factors that vary from probe to probe in a complete plant or system, will be taken into account and corrected for during the calibration, that takes place by suitable programming of the microprocessor or computer.
This also comprises calibration or compensation for component aging in the oscillator circuit with time and changes in the properties of the electrodes, such as by coating that by and by can attach thereto, possibly in the form of fouling.
The above discussed, specific features according to the invention make it possible to provide apparatuses of the kind stated here, having a high sensitivity and a high accuracy in detection and measurement, as well as various and highly different applications that have not hitherto been possible in practice with known forms of capacitive detection or measurement. For example, it is possible to obtain measurements of as small a proportion as 0,1% of water in oil. Moreover, the apparatus according to the invention has the advantage of consuming very little electric power, namely at the order of magnitude of 1 mW in typical embodiments.